Thursday Morning: New Current Discovered, and a Leg Up for Tiny Trichodesmium

The new current is called the Antarctic Peninsula Coastal Current (blue). It flows inshore of, and opposite to, the well-known Antarctic Coastal Current (red). Moffat and colleagues don't yet know how the current flows past Marguerite Bay (question mark). (Carlos Moffat, WHOI)

Trichodesmium is a single-celled, rod-shaped organism (below) important in the marine food chain.
Individuals clump into groups shaped like lint balls (top) that can be up to 4 mm (1/8 inch) across. (John Waterbury, WHOI)

Related Links

» Voyages into the Antarctic WinterDetails about the super-productive Antarctic waters, from the research cruise on which Moffat gathered his data. From Oceanus magazine.

This Just In: Two More Amazons

Sailors have been mapping ocean currents since they invented sails, so I was surprised to hear yesterday that a new one had just been discovered.

Carlos Moffat, an MIT/WHOI Joint Program student, and his advisers, Robert Beardsley and Breck Owens, named it the Antarctic Peninsula Coastal Current for the 1,200-km (750-mile) spit of rock and glaciers that stretches toward the tip of South America.

The current pumps about 0.3 Sverdrup of water - almost two Amazon Rivers worth - per year southward along the peninsula’s west coast. The strongest part of the current is 10 kilometers wide and about 200 meters deep, and has a top speed of about 35 cm/sec (that's 6 miles, 660 feet, and 1 foot/sec).

Moffat and colleagues discovered the current after placing current sensors underwater for an entire year in 2001-2002. He says the new current is driven by fresh water - precipitation and runoff from the region’s many melting glaciers. As that fresh water runs out to sea, it hits a wall of much saltier ocean water and makes a left (southward) turn, forming the current.

It’s understandable that this current went undiscovered for so long, since it lies only 20 kilometers off Antarctica and is covered by sea ice each winter. And Moffat’s instruments indicate the current shuts off each winter as coastal water freezes, leaving the remaining water much saltier. When spring brings melting temperatures, the current heaves back into action.

Moffat says his interests lie in biology, but he became a physical oceanographer because he wanted to explain why certain regions of the ocean - like the Antarctic Peninsula - are so rich in marine life. This current may encourage the conditions for tiny krill - and the seals, whales and penguins that eat them - to thrive.

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In the Cutthroat World of Mid-Ocean Microbes, Trichodesmium Holds the Edge

The ocean surface is a soup of tiny creatures trying to make it through their lives, often without enough nitrogen, iron, or phosphorus to go around. In this cutthroat world, one organism, a cyanobacterium called Trichodesmium, has a newfound advantage.

The colonial, rod-shaped cells can harvest phosphorus from an organic material called phosphonate that was previously thought off-limits. Biologist Sonya Dyhrman, speaking for the WHOI research team that did the work, announced today (and in an article last month in the journal Nature).

Together, Trichodesmium and its competitors are the tiny photosynthetic organisms called phytoplankton. Using the sun’s energy, carbon dioxide, and some nutrients, they make the food that feeds virtually the entire ocean.

The organisms strip the nutrients they need from larger molecules. But chemistry is difficult even for microorganisms, and some nutrients are simply inaccessible because the phytoplankton can’t break certain chemical bonds.

Dyhrman’s team studied Trichodesmium genetics in relation to phosphorus use in the Sargasso Sea, east of the Caribbean. Trichodesmium can be abundant there, despite very low levels of phosphorus in the water. Blooms of Trichodesmium have tinted the water red over areas as large as New Mexico, 120,000 square miles (300,000 square kilometers).

The WHOI scientists scanned an entire sequence of Trichodesmium DNA and found a suite of genes that can extract phosphorus from phosphanate. Those genes were absent from the DNA of Trichodesmium’s close competitors.

But simple organisms swap genes frequently, by accident, and don't necessarily use their new acquisitions. Dyhrman's next experiment verified that Trichodesmium used the genes to add working enzymes to their cellular toolbox.

The finding helps scientists like Dyrhman and her colleagues understand how Trichodesmium can reach such abundance in waters with very low nutrient levels. Of course, that still leaves the question of how the organism gets its iron.